Method for materials recovery from catalysts comprising iron, cerium, molybdenum, and potassium

ABSTRACT

A method for materials recovery from a catalyst comprising oxides of iron, cerium, molybdenum, and potassium, in which potassium and molybdenum are removed by treating the catalyst with an aqueous leachant, giving an aqueous solution S1 comprising potassium and molybdenum, and a solid residue R1 comprising cerium oxide and iron oxide, and recovering cerium in the form of a solid comprising a cerium(III) compound or cerium(IV) oxide from the solid residue R1.

The invention relates to a method for materials recovery from catalysts comprising iron, cerium, molybdenum, and potassium.

In view of sharply increased prices for rare earth compounds, there is a strong interest in the targeted recovery of rare earth compounds from spent catalysts. For the recovery of cerium compounds, attention is focused on spent iron oxide catalysts of the kind used for dehydrogenating hydrocarbons, as for example for dehydrogenating isopentene to isopentadiene (isoprene) or ethylbenzene to styrene. These catalysts generally have a cerium oxide content in the range from 1% to 25% by weight, more particularly from 5% to 15% by weight. Catalysts of this kind are described for example in EP 1 027 028 A1, DE 101 54 718 A1, and EP 0 894 528 B1.

The sometimes complex composition of such catalysts is a challenge in their workup. For instance, alongside compounds of iron and cerium, the catalysts also comprise compounds of potassium and molybdenum, and, possibly, compounds of magnesium, calcium, tungsten, titanium, copper, chromium, cobalt, nickel, and vanadium.

A particular challenge is posed by the molybdenum content of the catalysts. Following removal of cerium, the presence of molybdenum prevents wastewater disposal of the solutions that comprise otherwise substantially unproblematic elements such as iron, potassium, magnesium, and calcium. In the absence of molybdenum, moreover, it would be possible to use a solution comprising iron, potassium, magnesium, and calcium effectively for neutralization or as a flocculant in wastewater treatment plants.

Moreover, high levels of potassium are disruptive to the workup of the iron-containing solutions, which remain following removal of cerium, in plants of the kind operated by steel manufacturers for the processing of hydrochloric acid pickling solutions, for the purpose of recovering iron oxide (by the process known as the Ruthner process, for example). A process of this kind for preparing iron oxides from iron chloride solutions containing hydrochloric acid is described for example in EP 0 850 881 A1.

KR2002-0093455 describes a method for recovering cerium dioxide from spent dehydrogenation catalysts which are used for the manufacture of styrene from ethylbenzene. There, the spent catalyst is subjected to wet grinding, in a ballmill, for example, down to a particle size of 0.1 to 5 μm. Following removal of the ground catalyst by filtration, and addition of acid to the solid, a slurry having a pH of 0.5 to 3.0 is provided, and is filtered again. Water is added to the solid residue which, with addition of organic dispersants, is dispersed using ultrasound, after which two phases are formed, the lower phase being said to comprise iron oxide (magnetite) and the upper phase to comprise cerium oxide. The very fine grinding of the spent catalyst is costly and inconvenient; the organic dispersants used may pollute the wastewater. The isolation of molybdenum is not mentioned.

WO 2007/009927 A1 describes a process for preparing dehydrogenation catalysts using secondary raw material obtained by workup of spent catalysts. With this process, there is no chemical separation of the catalyst constituents into compounds of the individual metals. Instead, 10% to 70% by weight of a calcined and ground spent catalyst comprising iron oxide is mixed with 30% to 90% by weight of a fresh catalyst material comprising iron oxide.

It is an object of the invention to provide a simple method for materials recovery from spent dehydrogenation catalysts which as well as iron and cerium also comprise molybdenum and potassium. A particular object of the invention is to provide a method in which disruptive molybdenum is isolated in a simple way and the major further components, cerium and iron, are supplied for materials recovery. Another object of the invention is to provide a method in which molybdenum as well is supplied for materials recovery.

The object is achieved by means of a method for materials recovery from a catalyst comprising oxides of iron, cerium, molybdenum, and potassium, in which potassium and molybdenum are removed at least partly by treating the catalyst with an aqueous leachant, giving an aqueous solution comprising potassium and molybdenum, and a solid residue comprising cerium oxide and iron oxide, and recovering cerium in the form of a solid comprising a cerium(III) compound or cerium(IV) oxide from the residue.

Generally speaking, the spent catalysts which the invention subjects to materials recovery comprise the following metals:

-   iron, corresponding to 40% to 90% by weight Fe₂O₃, -   potassium, corresponding to 1% to 40% by weight K₂O, -   molybdenum, corresponding to 0.1% to 10% by weight MoO₃, -   cerium, corresponding to 1% to 25% by weight CeO₂, more particularly     5% to 15% by weight CeO₂, -   calcium, corresponding to 0% to 10% by weight CaO,

The catalysts often also comprise magnesium, corresponding to 0% to 10% by weight MgO.

Furthermore, the catalysts may, in small amounts, also comprise further elements present as typical impurities in the iron oxides used primarily.

In one embodiment of the method of the invention, the catalysts comprise oxides of iron, potassium, molybdenum, cerium, and calcium. In another embodiment, the catalysts comprise oxides of iron, potassium, molybdenum, cerium, calcium, and magnesium.

An essential step in the method of the invention is the leaching of the spent catalyst with an aqueous leachant, to give an aqueous alkaline solution S1 comprising molybdenum and potassium, and a solid residue R1 comprising oxides of cerium and iron. Generally speaking, the solid residue further comprises oxides of calcium and magnesium and also, possibly, the oxides of other metals.

In accordance with the invention, both potassium and molybdenum are isolated from the rest of the components with particular simplicity from the spent catalyst material by simple leaching or extracted washing with an aqueous leachant, preferably hot water. Potassium and molybdenum are therefore unable to disrupt the recovery of materials from the rest of the components. The leaching step isolates generally >90%, more particularly >95%, of the molybdenum present, and >50%, more particularly >70%, of the potassium present.

The aqueous leachant may comprise alkaline additions or impurities. Suitable alkaline additions are alkali metal hydroxides and carbonates, alkaline earth metal hydroxides and carbonates, ammonia, amines or carboxylates. In one preferred embodiment of the method of the invention, the aqueous leachant comprises no alkaline additions. The aqueous leachant may also comprise weakly acidic additions or impurities, examples being carbonic acid or carboxylic acids. In one preferred embodiment of the method of the invention, the aqueous leachant contains no acidic additions. Generally speaking, the leaching of the spent catalyst is carried out at a temperature of at least 50° C., preferably at not less than 70° C., more preferably at not less than 90° C., under atmospheric pressure or superatmospheric pressure. Implementation under superatmospheric pressure may be advantageous in order to attain temperatures of around 100° C. or more. Generally speaking, the temperature is not more than 150° C., preferably not more than 110° C., more preferably not more than 100° C. Generally speaking, implementation with substantial increase in pressure is unnecessary for carrying out the method of the invention. Leaching is carried out more particularly under atmospheric pressure and at temperatures from 50 to 100° C.

Generally speaking, potassium and molybdenum in the aqueous solution S1 obtained are present in the form of a mixture of different compounds. At least some of the potassium and some of the molybdenum in the aqueous solution obtained can be specified formally as dissolved potassium molybdate (K₂MoO₄), potassium carbonate (K₂CO₃) or potassium hydroxide (KOH). The aqueous solution obtained, comprising potassium and molybdenum, has a pH of generally 7 to 15, preferably 8 to 15, more particularly 10 to 14.

The leaching of the spent catalysts may be carried out in one or more stages, continuously or discontinuously. In general it will be advantageous to minimize the volume of the aqueous solution S1, comprising potassium and molybdenum, with an eye to the further workup of the solution, and nevertheless to achieve very substantial transfer of molybdenum and potassium into the aqueous solution S1. In order to be able to operate with a very low solution volume, it is possible, for example, to operate multiply in a plurality of batches or sequentially in a multistage continuous process with relatively small solution volumes, which can be combined for further workup. Leaching may for example be carried out continuously in accordance with the countercurrent principle or sequentially (in multistage form) in accordance with the countercurrent principle. In this case, the more weakly concentrated wash solutions produced in later leaching steps are used as the leaching solution in earlier leaching steps. It is also possible for the mother liquor which remains following the removal of molybdenum by precipitation to be employed as the leaching solution, at least in some of the stages of a multistage leaching process.

The leaching of the spent catalyst with the aqueous leachant can be carried out for example by slurrying the spent catalyst in a vessel and subsequently removing the aqueous leaching solution by decanting or filtration. A vessel of this kind preferably possesses a stirrer or another means of circulating the leaching slurry, such as a circulation pump, for example. A vessel of this kind may be heated indirectly, as for example by steam-fed heating coils, or else directly, as for example by direct feeding of steam into the slurry. A vessel of this kind can also be operated without heating, if, for example, a correspondingly preheated aqueous solution is used in the vessel.

The aqueous solution can be removed from the vessel either by decanting or filtration. If the catalyst is to be leached a number of times in succession, it can be advantageous to carry out decanting, at least in the first stages. Generally speaking, however, it is advantageous to filter the leached slurry. This can be done, for example, using filter presses or belt filter units. If filter presses are utilized, it can be advantageous to wash the filtercake with an aqueous solution.

If belt filters are used, it can be advantageous to combine filtration with leaching or washing steps. For example, a belt filter can be used in which different aqueous solutions can be applied sequentially in spatially delimited belt sections, for leaching or for washing; in this case, more weakly concentrated solutions from later sections of the belt filter can be applied as leaching solutions in earlier sections of the belt filter, in order to keep the overall solution volume as small as possible.

The inventive step of leaching with aqueous leachant may be preceded by a step in which the spent catalyst is wholly or partly mechanically comminuted—that is, for example, shredded or ground.

Leaching with aqueous leachant may also be preceded by a method step in which the spent catalyst is heated in an atmosphere comprising oxygen, and so the iron oxides present in the catalyst are converted substantially into the +III oxidation state, and the cerium compounds present therein are converted into the +IV oxidation state. In this case, also, any carbon-containing deposits present are burnt off.

In one preferred embodiment of the method of the invention, the catalyst, before being treated with the aqueous leachant, is heated in an atmosphere comprising oxygen.

In another embodiment of the method of the invention, the catalyst, before being treated with the aqueous leachant, is mechanically comminuted.

Although it is generally advantageous to carry out the heating of the catalyst in the atmosphere comprising oxygen prior to the mechanical comminution step, an inverted sequence is also possible in principle.

The thermal treatment of the spent catalyst in the presence of oxygen is carried out generally at temperatures of 100 to 1200° C., preferably of 200 to 1000° C., more preferably of 300 to 1100° C. and more particularly of 300 to 1000° C. Generally speaking, the catalyst is heated (calcined) in the oxygen-containing atmosphere for a period of 30 minutes to 10 hours, preferably of 1 to 3 hours. The iron present in the spent catalyst material, after calcining, is generally in the form, substantially, of hematite, magnetite, and potassium ferrite phases. The cerium dioxide (CeO₂) present in the catalyst material generally, after calcining, has crystallite sizes of 15 to 90 nm, more particularly of 40 to 60 nm.

It is preferred to use air as oxygen-containing gas. Lean air can also be used.

Calcining may be carried out discontinuously or continuously, in tray ovens or rotary tubes, for example. The thermal treatment is preferably carried out continuously in rotary tubes. Especially in the case of catalyst material which is highly pulverized or has a high powder content, it can be useful to carry out calcining in a rotary tube equipped with tappers.

In one variant, the heating of the spent catalyst may be carried out while it is still in the dehydrogenation reactor, before being deinstalled. For this purpose, after flushing has taken place with nitrogen or steam, air or an oxygen-containing gas is fed into the dehydrogenation reactor.

After the thermal treatment, optionally, the spent catalyst material is preferably ground in suitable mills. It may be advantageous to carry out initial preliminary crushing of the material. This preliminary crushing may take place, for example, in a cam crusher. The material can subsequently be comminuted to completion using, for example, a hammer mill. The average particle diameter is in the range from 1 to 700 μm, preferably 5 to 500 μm, more particularly from 10 to 200 μm. Generally speaking, however, the method of the invention can also be carried out with relatively coarse material or with uncomminuted material.

From the solid residue which remains after leaching and which comprises oxides of cerium and iron and also, generally speaking, oxides of calcium and possibly magnesium or other metals as well, cerium is recovered in the form of a solid comprising a cerium(III) compound or cerium(IV) oxide.

The invention accordingly provides a method for materials recovery from a spent catalyst comprising oxides of iron, cerium, molybdenum, and potassium, with the following steps:

-   (i) optionally heating the catalyst in an oxygen-containing     atmosphere, -   (ii) optionally mechanically comminuting the catalyst, -   (iii) removing potassium and molybdenum by treating the catalyst     with an aqueous leachant, giving an aqueous solution S1 comprising     potassium and molybdenum, and a solid residue R1 comprising cerium     oxide and iron oxide, -   (iv) recovering cerium in the form of a solid comprising a     cerium(III) compound or cerium(IV) oxide from the residue R1, -   (v) optionally recovering iron in the form of iron oxide from the     residue R1, -   (vi) optionally recovering molybdenum in the form of an     oxygen-containing molybdenum(VI) compound from the aqueous solution     S1 comprising potassium and molybdenum.

In one embodiment of the invention, cerium is recovered in the form of a solid comprising cerium(IV) oxide. In a preferred embodiment, the solid residue comprising cerium oxide and iron oxide is treated with a mineral acid, to give an iron-containing solution S2 and a solid residue R2 containing cerium(IV) oxide. The resulting residue may comprise other metal oxides as well as cerium(IV) oxide, more particularly iron oxide, and so it may be necessary to refine the solid (residue R2) containing cerium(IV) oxide. In this case, from the cerium(IV) oxide, it is possible, by techniques which are common knowledge, to prepare further cerium compounds, without the cerium dioxide necessarily having to be purified beforehand.

A preferred mineral acid is hydrochloric acid. The hydrogen chloride content of the hydrochloric acid is generally from 5% to 37% by weight. It is also possible in principle, however, to use other acids, as for example nitric acid or aqueous sulfuric acid. The pH of the acidic solution may go up with increasing dissolution of constituents of the solid. Ultimately the pH of the solution S2 is generally between 0 and 7, preferably between 0.5 and 6, and with more particular preference between 1 and 5.

The treatment (the leaching) of the solid residue with mineral acid may be carried out in one or more stages, discontinuously or continuously, in a cocurrent or countercurrent regime. Suitable variants are described in Ullmann's Encyclopedia of Industrial Chemistry, Liquid-Solid Extraction chapter (T. Voeste et al.). In the case of a countercurrent treatment with a multistage leaching procedure, the more weakly concentrated leaching solutions from later leaching steps are used as the leaching solution in earlier leaching steps. The treatment is carried out in particular at elevated temperatures, preferably at 30 to 120° C., more particularly 50 to 100° C. Treatment may be carried out under atmospheric pressure or increased pressure. Generally speaking, however, there is no need for an increase in pressure. The treatment with acid may be carried out in a tank, for example. Such a tank preferably possesses a stirrer or other means of circulating the leaching slurry, such as a circulation pump, for example. A tank of this kind may be heated preferably indirectly, as for example by steam-fed heating coils, or else directly, as for example by direct feeding of steam into the slurry. A tank of this kind can also be operated without heating if a preheated mineral acid is used in the tank.

The acidic solution can be removed from the tank either by decanting or filtration. If the catalyst is to be leached a number of times in succession, it can be advantageous to carry out decanting, at least in the first stages. Generally speaking, however, it is advantageous to filter the leached slurry. This can be done using, for example, filter presses or belt filter units. If filter presses are utilized, it can be advantageous to wash the filtercake with an aqueous solution.

In one variant of this embodiment, the treatment step with mineral acid can also be carried out in the presence of an oxidizing agent. Suitable oxidizing agents may be nitric acid or perchloric acid, for example. This may be especially advantageous when the catalyst is not calcined before being treated with aqueous leachant.

From the iron-containing solution S2, iron can be precipitated in the form of iron hydroxide or iron oxide by addition of a basic precipitant. Suitable basic precipitants are alkali metal or alkaline earth metal hydroxides.

In another embodiment of the method of the invention, iron oxide is recovered from the iron-containing solution S2 by spray drying.

The solution S2, comprising iron and, generally speaking, calcium and possibly magnesium as well, can also be introduced as a flocculant into a wastewater treatment plant. This is possible in particular because the solution contains substantially no molybdenum.

The iron-containing solution S2 can also be used for preparing iron oxides, which can be used in turn for producing pigments, ferrites or catalysts.

For example, iron oxides of this kind can be prepared by spray roasting of the hydrochloric acid solutions. The hydrochloric acid solution S2 can also be added to a pickling solution obtained in steelmaking. From this solution, iron oxides and hydrochloric acid can be obtained in a spray roasting procedure (Ruthner process). This type of workup is possible especially when the solution does not contain much potassium.

Alternatively, iron oxides or iron hydroxides can also be obtained by precipitation using bases from the solution S2.

The invention also provides, therefore, a method for materials recovery from a spent catalyst comprising oxides of iron, cerium, molybdenum, and potassium, with the following steps:

-   (i) optionally heating the catalyst in an oxygen-containing     atmosphere, -   (ii) optionally mechanically comminuting the catalyst, -   (iii) removing potassium and molybdenum by treating the catalyst     with an aqueous leachant, giving an aqueous solution S1 comprising     potassium and molybdenum, and a solid residue R1 comprising cerium     oxide and iron oxide, -   (iv) recovering cerium in the form of a solid comprising cerium(IV)     oxide from the solid residue R1, by treating the solid residue with     a mineral acid, to give an iron-containing solution S2 and a solid     residue R2 comprising cerium(IV) oxide, -   (v) optionally recovering iron(III) oxide, by spray roasting, for     example, from the iron-containing solution S2, -   (vi) optionally recovering an oxygen-containing molybdenum(VI)     compound from the aqueous solution S1 comprising potassium and     molybdenum.

In a further embodiment of the invention, cerium is obtained in the form of a cerium(III) compound. In one embodiment of the method of the invention, the solid residue R2 comprising cerium oxide is fully dissolved in a mineral acid in the presence of a reducing agent, in the course of which cerium(IV) is reduced to cerium(III), and a solution S4 is obtained. By reduction of the inherently insoluble cerium(IV) oxide by addition of a reducing agent, cerium(IV) oxide is converted into soluble cerium(III). Cerium can be precipitated from this solution S4 in the form of a cerium(III) compound, more particularly a cerium(III) carbonate hydrate, by addition of a basic precipitant.

A preferred mineral acid is hydrochloric acid, generally with a concentration of 5% to 37% by weight hydrogen chloride, or aqueous sulfuric acid.

For reducing cerium(IV) to cerium(III) it is preferred to add to the mineral acid, as reducing agent, an iron component in which iron is present in the form of metallic iron, in the 0 oxidation state, or in the form of iron(II). The iron component can be added in a solid form, as iron powder, for instance, or in the form of an iron(II) salt solution. The iron component is preferably selected from the group consisting of elemental iron, iron alloys, steel (e.g. steel filings), steel alloys, iron(II) chloride, iron(II) carbonate, iron(II) sulfate, and iron(II) ammonium sulfate. One embodiment of the invention uses iron scrap (e.g., iron scrap filings) as the iron component.

The iron component is added in the 0 and/or II oxidation state, in order to convert the existing cerium(IV) into the soluble cerium(III) form. The iron component is used more particularly in a stoichiometric proportion, relative to cerium.

If an iron component containing iron in the 0 oxidation state is added, hydrogen gas may be evolved and the mixture may undergo foaming. In this embodiment, in particular, suitable engineering measures can be taken to remove the hydrogen gas. Also possible is the addition to the mixture of defoamers, as for example of defoamers based on silicone oil or polydimethylsiloxanes.

Generally speaking, the dissolving of the solid residue comprising cerium oxide and iron oxide in the mineral acid takes place at a temperature of 40 to 160° C., preferably of 40 to 120° C., more preferably of 60 to 120° C., and more particularly of 90 to 120° C. The dissolution takes place more particularly at the boiling point of the mixture, preferably under reflux. The dissolving operation generally takes 10 minutes to 8 hours, preferably 0.5 to 4 hours, as for example 0.5 to 2 hours.

Subsequently, cerium can be precipitated from the resulting solution, and by addition of a basic precipitant, in the form of a cerium(III) compound.

A cerium(III) compound is generally precipitated by addition of a basic precipitant to the mineral acid solution until a pH in the range from 6 to 12, preferably from 7 to 9, is reached.

Cerium(III) is precipitated generally in the form of cerium(III) carbonate hydrate. Suitable basic precipitants are alkali metal carbonates and alkaline earth metal carbonates; sodium carbonate is preferred.

Where an iron compound is added for reducing the cerium(IV) dioxide, the iron compound can be precipitated either fractionally, by means of pH control, or jointly with the cerium(III) compound. Joint precipitation may be advantageous especially when the cerium compound is to be used for producing catalysts comprising oxides of iron, cerium, molybdenum, and potassium.

The invention also provides, therefore, a method for materials recovery from a spent catalyst comprising oxides of iron, cerium, molybdenum, and potassium, with the following steps:

-   (i) optionally heating the catalyst in an oxygen-containing     atmosphere, -   (ii) optionally mechanically comminuting the catalyst, -   (iii) removing potassium and molybdenum by treating the catalyst     with an aqueous leachant, giving an aqueous solution S1 comprising     potassium and molybdenum, and a solid residue R1 comprising cerium     oxide and iron oxide, -   (iv) recovering cerium in the form of a solid comprising cerium(IV)     oxide from the solid residue R1, by treating the solid residue with     a mineral acid, to give an iron-containing solution S2 and a solid     residue R2 comprising cerium(IV) oxide, -   (v) dissolving the solid residue R2 in a mineral acid, with     reduction of cerium(IV) to cerium(III), giving a solution S4, -   (vi) fractionally or jointly precipitating the cerium(III) compound     and, optionally, dissolved iron compounds by adding a basic     precipitant to the solution S4, -   (vii) optionally recovering an oxygen-containing molybdenum(VI)     compound from the aqueous solution S1 comprising potassium and     molybdenum.

Molybdenum can be precipitated from the aqueous solution containing potassium and molybdenum by addition of an acid, in the form of oxygen-containing molybdenum(VI) compound (“molybdenum(VI) oxide”).

In one preferred embodiment of the invention, the solution S1 comprising potassium and molybdenum, in a further method step, is admixed with an acid—sulfuric acid, hydrochloric acid or nitric acid, for example—for neutralization or, furthermore, for acidification to an extent such that a solid R3 comprising an oxygen-containing molybdenum(VI) compound is precipitated, leaving a solution S3 comprising potassium salts. The oxygen-containing molybdenum(VI) compound is generally precipitated at a pH of 2 to 7. The precipitation product comprising molybdenum oxide can be worked up further or used directly for producing molybdenum-containing materials. The solution comprising potassium salts, following removal of the predominant amount of molybdenum, can generally be disposed of without problems.

If the iron-containing acidic solution S2 in the method of the invention is to be used in a way in which the presence of potassium is not an interference, as a flocculant in wastewater treatment plants, for example, it may also be advantageous to use the potassium-containing solution obtained following removal of the molybdenum again, in the treatment of the solid R1 comprising iron and cerium.

The solid R3 may also comprise potassium. In one variant of the method of the invention, the solid R3 can also be used for producing catalysts comprising oxides of iron, cerium, molybdenum, and potassium, without any need for removal of potassium beforehand.

Also possible is the spray drying of the solution comprising potassium and molybdenum to give a solid comprising potassium and molybdenum, after saturation with carbon dioxide, for example.

The invention is illustrated by the examples below.

EXAMPLES

The examples illustrate the invention, using exemplary embodiments of individual method steps on a laboratory scale. An optimization of these method steps, with the aim, for example, of optimizing the solution volumes, the energy input or the amount of the individual constituents in the solutions or solids, is readily possible for the skilled person, taking account of general principles and the particular circumstances.

Example 1

A spent catalyst comprising iron oxide, potassium, molybdenum, cerium, calcium, and magnesium from a plant for the dehydrogenation of ethylbenzene was calcined in a laboratory oven at 800° C. for 2 hours, and ground using an analytical mill.

205 g of this catalyst were stirred with 410 ml of fully deionized water (“DI water”) at 95° C. for 30 minutes (glass beaker with magnetic stirrer bar on heating stirrer). The pH of the solution was 12 at 95° C. The solution was filtered through a paper filter in a ceramic suction filter attachment, and the filtercake was washed with 45 ml of hot DI water. The combined clear, yellow solutions (solution 1-1) gave a pH of 13.4 after cooling to 39° C.

The filtercake was stirred again with 400 ml of DI water at 95° C. for 30 minutes. The pH at 95° C. was 11.3. The solution was filtered through a paper filter on a ceramic suction filter attachment, and the filtercake was washed with 80 ml of hot DI water. The combined clear, colorless solutions (solution 1-2) gave a pH of 12.5 after cooling to 42° C.

The filtercake was slurried with 150 ml of boiling DI water and filtered through a paper filter on a ceramic suction filter attachment, and the filtercake was washed with 100 ml of boiling DI water. The clear, colorless solution (solution 1-3), combined from filtrate and wash water, gave a pH of 11.9 after cooling to 38° C.

This gave 240 g of moist filtercake (solid 1-1) with a loss on ignition (900° C.) of 28.7%.

Table 1 shows the analytical results for Example 1.

Example 2

318.6 g of solution 1-1 from Example 1 were admixed with dilute sulfuric acid and left to stand at room temperature. Overnight, a white, readily filterable solid had deposited, which was filtered off via a suction filter attachment with paper filter. This gave 37.7 g of moist filtercake (solid 2-1) with a loss on ignition (900° C.) of 87.5% and 506.6 g of residual solution (solution 2-1) with a pH of 1.7.

Table 1 shows the analytical results for Example 2.

Example 3

216 g of the moist filtercake (solid 1-1) from Example 1 were stirred with 1.4 l of a mixture of DI water and hydrochloric acid (pH 0) at 80° C. for an hour (glass beaker with magnetic stirrer bar on heating stirrer). The suspension was subsequently filtered through a paper filter on a ceramic suction filter attachment, and the filtercake was washed with 150 ml of concentrated hydrochloric acid. This gave 1543 g of solution (solution 3-1) and 16.29 g of a moist filtercake (solid 3-1).

Table 1 shows the analytical results for Example 3.

The % figures are % by weight. The figures for solids are based on the calcined solids.

TABLE 1 Example 1 Example 2 Example 3 Solution 1-1 Solution 2-1 Solution 3-1 Total  100% 342.2 g Total  100% 506.6 g Total  100% 1543 g Fe2O3   0% 0 g Fe2O3   0% 0.00 g Fe2O3 7.80% 120.35 g K2O  4.0% 13.69 g K2O 2.31% 11.70 g K2O 0.33% 5.09 g CeO2  0.0% 0 g CeO2  0.0% 0.00 g CeO2 0.00% 0.00 g CaO  0.0% 0 g CaO  0.0% 0.00 g CaO 0.18% 2.78 g MgO  0.0% 0 g MgO  0.0% 0.00 g MgO 0.19% 2.93 g MnO2  0.0% 0 g MnO2  0.0% 0.00 g MnO2 0.03% 0.46 g MoO3  1.0% 3.42 g MoO3 0.04% 0.20 g MoO3 0.00% 0.00 g Solution 1-2 Total  100% 449 g Fe2O3   0% g K2O 0.56% 2.51 g CeO2 0.00% g CaO 0.00% g MgO 0.00% g MnO2 0.00% g MoO3 0.09% 0.40 g Solution 1-3 Total  100% 233 g Fe2O3   0% 0 g K2O 0.12% 0.28 g CeO2  0.0% 0 g CaO  0.0% 0 g MgO  0.0% 0 g MnO2  0.0% 0 g MoO3  0.0% 0 g Solid 1-1 Solid 2-1 Solid 3-1 Moist 240 g Moist 37.7 g Moist 16.29 g LOI 900 28.70%  68.88 g LOI 900 87.50%  32.988 g LOI 900 14.3% 2.329 g  100% 171.12 g  100% 4.713 g  100% 13.961 g Fe2O3 82.00%  140.32 g Fe2O3 0.01% 0.00 g Fe2O3 4.10% 0.57 g K2O 3.40% 5.82 g K2O 22.60%  1.07 g K2O 0.00% 0.00 g CeO2 11.10%  18.99 g CeO2 0.00% 0.00 g CeO2 93.00%  12.98 g CaO 2.30% 3.94 g CaO 0.00% 0.00 g CaO 0.45% 0.06 g MgO 2.30% 3.94 g MgO 0.05% 0.00 g MgO 0.02% 0.00 g MnO2 0.37% 0.63 g MnO2 0.40% 0.02 g MnO2 0.03% 0.00 g MoO3 0.08% 0.14 g MoO3 79.00%  3.72 g MoO3 0.40% 0.06 g 

1.-11. (canceled)
 12. A method for materials recovery from a catalyst comprising oxides of iron, cerium, molybdenum, and potassium, in which potassium and molybdenum are removed by treating the catalyst with an aqueous leachant, giving an aqueous solution S1 comprising potassium and molybdenum, and a solid residue R1 comprising cerium oxide and iron oxide, and recovering cerium in the form of a solid comprising a cerium(III) compound or cerium(IV) oxide from the solid residue R1.
 13. The method according to claim 12, wherein the catalyst, before being treated with the aqueous leachant, is heated in an atmosphere comprising oxygen.
 14. The method according to claim 12, wherein the catalyst, before being treated with the aqueous leachant, is mechanically comminuted.
 15. The method according to claim 12, wherein the solid residue R1 comprising cerium oxide and iron oxide is treated with a mineral acid, to recover an iron-containing solution S2 and a solid residue R2 comprising cerium(IV) oxide.
 16. The method according to claim 15, wherein the solid residue R2 comprising cerium(IV) oxide is dissolved, with reduction of cerium(IV) to cerium(III), in a mineral acid, giving a solution S4, and cerium is precipitated in the form of a cerium(III) compound from the solution S4 by addition of a basic precipitant.
 17. The method according to claim 16, wherein an iron(II) compound or metallic iron is added as reducing agent to the mineral acid.
 18. The method according to claim 16, wherein an alkali metal carbonate or alkaline earth metal carbonate is used as basic precipitant, and cerium is precipitated in the form of cerium(III) carbonate hydrate.
 19. The method according to claim 15, wherein iron is precipitated in the form of iron oxide or iron hydroxide from the iron-containing solution S2 by addition of a basic precipitant.
 20. The method according to claim 15, wherein iron oxide is recovered from the iron-containing solution S2 by spray drying.
 21. The method according to claim 15, wherein the iron-containing solution S2 is introduced as a flocculant into a wastewater treatment plant.
 22. The method according to claim 12, wherein molybdenum is precipitated in the form of a solid R3 comprising an oxygen-containing molybdenum(VI) compound from the aqueous solution S1 comprising potassium and molybdenum by addition of an acid, leaving a solution S3 comprising potassium salts. 